Time-Resolved Raman Spectroscopy of Polaron Formation in a Polymer Photocatalyst

Polymer photocatalysts are a synthetically diverse class of materials that can be used for the production of solar fuels such as H2, but the underlying mechanisms by which they operate are poorly understood. Time-resolved vibrational spectroscopy provides a powerful structure-specific probe of photogenerated species. Here we report the use of time-resolved resonance Raman (TR3) spectroscopy to study the formation of polaron pairs and electron polarons in one of the most active linear polymer photocatalysts for H2 production, poly(dibenzo[b,d]thiophene sulfone), P10. We identify that polaron-pair formation prior to thermalization of the initially generated excited states is an important pathway for the generation of long-lived photoelectrons.

front face of the sample. The Raman and PL signals were collimated and passed through a long pass filter (633 nm, Semrock RazorEdge LP02-633RE), which acts to block Rayleigh scattering, before reaching the Kerr-gate. The Kerr-gate consists of a Kerr-medium (2 mm path length quartz cell filled with CS2) and 2 crossed polarisers (25 mm aperture, Halbo Optics PS25). Activation of the Kerr-gate was achieved using a gating pulse (800 nm, 240 μJ, 2 ps, 2.4 W, 10 kHz), which was produced by a ps arm of the ULTRA Laser System and was polarised at 45 ° with respect to the polarisers and was focused before the Kerr-medium.
When the Kerr gate is active, the rotated polarization of the Raman signal enables it to pass through the output polariser, whislt the majority of the PL (the polarization of which has not been changed) is rejected. The 770 nm short-pass filter (Semrock FF01-770/SP) was placed after Kerr gate to block the scatter of 800 nm gating beam. The Raman scatter was disperesed via a spectrograph (Andor Shamrock 303i) and detected using a CCD camera (iDus DU-420A-BU2, Andor). Timing of the gating, probe and pump pulses was changed using optical delay lines (IMS-600LM, 600 mm travel range and UTS100CC, 100 mm travel range, both from Newport). The Raman shift was calibrated against the Raman spectra obtained for toluene and acetonitrile Spectra were collected and averaged over 3 repeats, each with an acquisition time of between 20 s (for short runs to generate kinetic traces) and 60 s (for generation of higher signal:noise spectra). Delay times were randomised during each scan to minimise systematic effects. For experiments using a water:methanol:triethylamine solvent the sample was replaced periodically due to the build up of gas bubbles and a small number of delay times ( 16) was studied with each sample. Samples were prepared by sandwiching dry powders or powders with a drop of the water:methanol:triethylamine mixture between two CaF2 windows with a 300 μm spacer and loaded into a Harrick cell which was purged with N2 for about an hour before measurements were taken. Samples were measured in a near back-scattering geometry and the sample was continually rastered to minimise the effect of photoproduct build-up during the experiment.
The Kerr gate was successful in removing the majority of the pump (400 nm) induced PL, but there was still some present in the recorded spectra, see scheme 1 main text. To correct spectra (and to provide time-resolved PL data) a series of experiments was also recorded where the Raman probe laser pulse arrived 100 ps prior to the 400 nm pump and 800 nm Kerr gate pulses. These spectra were then subtracted from the TR 3 data at each gating pulse time delay using a home-made routine in Labview (National Instruments) that also generated the Raman difference spectra.

Transient absorption (TA) spectroscopy
TA spectra were recorded using a Harpia-TA spectrometer (Light Conversion) in an experimental configuration reported previously. 6 Briefly the system consists of a femtosecond laser syetsm (Pharos-SP-10W, Light Conversion) that has an output wavelength of 1028 nm and a pulse duration of ~180 fs at 10 kHz. 1 W of the 1028 nm output is used to generate 400 nm excitation pulses using a commercial OPA with second harmonic generation module (Orpheus and Lyra, Light Conversion). Experiments were carried out with a 400 nm excitation (beam diameter ca. 600 m at sample, power at sample ~ 500 W, effective pumping repetition rate of 5 kHz). The white light TA probe beam is generated by focusing 1028 nm light onto a CaF2 crystal within the Harpia spectrometer and focussed to ca. 400 m at the sample. The samples of P10 (2.4 mg) were suspended in toluene (10 ml). The solution was purged with Ar for 10 minutes, and then placed into a quartz cuvette (pathlength of 2 mm) which was sealed with a septa and flushed with Ar prior to filling. A thin film of aggregates formed on the quartz cuvette walls. The difference in probe beam transmission in the pumped/un-pumped state was measuredusing a NMOS detector (S3901, Hamamatsu), following dispersion by a spectrograpgh (Kymera 193i, Andor). Data was analysed using Carpetview software (Light Conversion).

Raman Microscopy/steady state details
Steady state Raman spectra were recorded on a Renishaw InVia Confocal Raman Microscope, using either 633 nm (~1.5 μm spot size at 0.875 mW) or 532 nm (~1.3 μm spot size at 2.5 mW) Raman laser probes. P10 powder was sandwiched between two CaF2 windows and wetted with a few drops of a 1:1:1 mixure of water:methanol:triethylamine.
Spectral acquisition involved 2 accumulations at 10 s exposure time. A 365 nm LED was mounted to the stage of the Raman microscope to illuminate the sample with a power of ~15 mW cm -2 during Raman acquisition. No significant background from scattered 365nm LED light was detected in the Raman spectra, allowing direct subtraction of spectra with and without illumination at the two laser wavelengths to plot the difference spectra shown in

Computational details
All density functional theory (DFT) 7,8 calculations were performed with Gaussian 16 (Revision A.03). 9 All structures correspond to minimum energy arrangements i.e., stationary points on the multi-dimensional potential energy surface, and subsequently validated through Preresonance Raman spectra for the neutral oligomers where the Raman excitation wavelength is much longer than the wavelength corresponding to the absorption onset were obtained by specifying the "Raman" option for the frequency keyword in Gaussian 16 in combination with adding the "ROA" option for the polar keyword and the wavelength of the probe (630 nm). 14 Resonance Raman spectra were calculated using the FC option in Gaussian 16 and involved the Frank-Condon approximation for the transition dipole, as well as the vertical gradient approximation. 15 Resonance Raman spectra were calculated for excitation wavelengths corresponding to the lowest 4 or 5 vertical excitations of one-electron oxidized and reduced oligomers, as well as for the experimental wavelength of the probe. Finally, unless stated otherwise all DFT predicted (pre)resonance Raman spectra have been scaled by the same factor, obtained by aligning the most intense peak at ~1600 cm −1 in the experimental and predicted preresonance Raman neutral P10 spectra.
The following exchange-correlation functionals were used in the DFT calculations B3LYP, [16][17][18] CAM-B3LYP 19 and ωB97XD, 20 in-conjunction with a cc-pVDZ basis set. For B3LYP and CAM-B3LYP dispersion interactions were accounted for by inclusion of Grimme's D3 dispersion model, 21 while with ωB97XD by default accounting for these interactions through a version of Grimme's D2 dispersion model. 22 The time-dependent extension to DFT (TD-DFT) 23 was used for the prediction of vertical excited states and compared to that predicted by equation-of-motion coupled cluster singles and doubles (EOM-CCSD). [24][25][26][27] The chain length of P10 modeled is denoted by P10x, where x is either 1, 3 or 6 representing the monomer, trimer and hexamer, respectively, while the one electron-reduced and one-electron oxidized species are denoted by (e -) and (h + ), respectively. suggests that the range-separated functionals, as expected, yield more similar vertical excitation energies to EOM-CCSD than those obtained by B3LYP. As the calculated vertical excitation energies correspond to poles of the dynamic molecular polarizability and because the intensity of Raman peaks finds it origin in the change of this polarizability for a particular vibrational mode it stands to reason that the range-separated functionals should be preferred over B3LYP for prediction of the (pre)resonance Raman spectra, especially since we want to compare our prediction directly to experiment.
We observe an excellent agreement between the experimental neutral P10 Raman spectrum and the predicted preresonance Raman spectra for all oligomers and functionals, especially after scaling. For P103(e -) and P106(e -) we observe a shift of the strong peak at around 1600 cm −1 to lower wavenumbers, in line with the shift observed experimentally when comparing the neutral and polaron P10 spectra. This shift is not observed for P103(h + ) and P106(h + ) as there the strong peak remains in approximately the same place as for the neutral species. This shift in the case of P103(e -) and P106(e -) and the absence of such a shift for P103(h + ) and P106(h + ) is, in combination with the fact that such a shift is observed experimentally, probably the most compelling theoretical evidence that the experimental polaron signal is due to an electron rather than a hole polaron. Figure S1. Ground state Raman spectrum (633 nm) of P10 powder (full spectral window).   The species associated spectra (SAS) generated for P10 aggregates in toluene are shown in figure S5b and they align with the proposed kinetic models. The SAS of compartments 1 and 2 both show stimulated emission peaks (< 600 nm) and a broad absorption assignable to a singlet exciton, but with differing peak widths and relative intensities, in-line with species 0 being an initially generated hot excitonic state. The SAS of compartment 2 shows a peak at ca. 642 nm is seen in the toluene/P10 data which is assigned to the P10 polaron pair, in excellent agreement with a past TA study on this polymer photocatalyst. 1         The time resolution of the TR 3 experiment is limited due to the pulse duration of the laser system (2 ps) means that 1 and A1 have significant uncertainties associated with them. For   Table S6 for the fitting parameters.  See Table S6 for the fitting parameters.